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Electricity and Magnetism The Sciences chapter 5 Electricity • If you walk across a rug and pick up a charge from the carpet, are you negatively or positively charged? Electricity describes charged particles • Rub a balloon with wool, picks up electrons • Rub glass with silk, loses electrons • Objects attract or repel when “electrically charged” • “static electricity” Figure 5-1 The two kinds of electrical charges. Opposite charges attract, while like charges repel. Electricity describes charged particles • These charged particles can be at rest (“static electricity”) • or they may be moving (“current electricity”) Ben Franklin & Electric Charge • 1746, 1st to use “negative” and “positive” in e’statics • 1752 famous kite experiment • Invented lightning rod Lightning • Result of charges that become separated in thunder clouds • Negative charge on cloud induces positive charge on ground • Discharge causes lights and heats air The movement of electrons • What were Franklin’s “negative fluids”? • Need to know atomic structure to understand electricity Draw an atom. Label its parts. What do the different parts do? • Protons • Neutrons • Electrons • Every atom has a positively charged nucleus surrounded by negatively charged electrons. • The electrons of all atoms are identical. Each has the same quantity of negative charge and the same mass. • Protons and neutrons compose the nucleus. Protons are 1800X more massive than electrons, but carry an amount of positive charge equal to the negative charge of electrons • Neutrons have slightly more mass than protons and have no net charge. Static Electricity Coulomb’s Law • The electric force between two charged particles varies directly as the product of their charges and inversely as the square of the separation distances. • force (newtons) = k x 1st charge x 2nd charge / distance2 Coulomb’s Law • …observed that if 2 electrically charged objects are moved farther away from each other, the force between them gets smaller, just like gravity. • If the distance between the 2 objects is doubled, the force decreases by a factor of 4. • (inverse square relationship) Electrical Field • A kind of “aura” or “force field” around every electric charge • Extends radially away from the proton and in opposite direction about the electron. …distinguish between conductors and insulators • Conductors • Insulators • Material that electrons are able to pass through • Metals, ionic solutions • Material that electrons do not through easily • Glass, wood, rubber, plastics Electrical potential and electric current • Movement of electric charge creates electric current • Charges move as current only when energy is supplied to them • Circuit • Switch • Voltage • Current If we use the water analogy… • Voltage = the pressure • Current = the rate of flow Voltage • • • • • The push that makes electrons move. 1.5 V “D” cell or 6 V lantern battery Higher voltage = greater push on electrons Water analogy Voltage causes current. Electric Current • Voltage creates current • Current is the amount of charge passing a point in a circuit in a second • Metric unit = Ampere (A) • Measures by an ammeter • Different devices often carry different amounts of current …distinguish between DC and AC • Direct current • Current moves in one direction • From dry cells or batteries • Alternating current • Pumped to us by Cobb EMC • Oscillates back and forth at 60 Hz • wall sockets Ohm’s Law • How is current related to voltage? • Direct relation between the two led to discovery of “resistance” • Voltage / Current = Resistance (V / I = R) Resistance • measures how hard it is for current to move through a conductor (unit = Ohm). • Easier for electrons to move through thick wires than thin wires • Light bulb filament, thin, high resistance, heats up and glows …distinguish between Series Circuits & Parallel Circuits • Only one path for current flow • Same amount of current thru entire circuit • Cheap string of decorative lights • Alternate paths for current flow • Current divides up among the paths • Wiring system for lights and elec outlets in homes & buildings Electrical Safety • • • • Fuses Circuit breakers Ground-fault interrupter “Atoms Family” ElecSafety link Electric Power • Power = energy used / time • Also calculated as product of current and voltage • Watts = amp’s x volts • Ex: 60W bulb draws .5 A on a 120V line 120W lamp draws 1A on a 120V line If a 120V line to a socket is limited (by a fuse) to 15A, will it operate a 1200W dryer? Solution… • If a 120V line to a socket is limited (by a fuse) to 15A, will it operate a 1200W dryer? • 1200W = A x 120V A = 10, yes Review Questions from chap 5: Electricity and Magnetism • Try discussion questions 1-11, p. 114. • Problems 2,3,5,6,7, p. 114. • Answers are posted in study guide.